Dimethylformamide, a widely used solvent in the chemical industry, has been prepared commercially by two processes. Current technology for the manufacture of dimethylformamide is based on the direct carbonylation of dimethylamine, i.e., the reaction of carbon monoxide with dimethylamine in the presence of a soluble catalyst. Although the process is commercial, there are difficulties with this process. These problems include: carbon monoxide production must be fairly local, since carbon monoxide is not easily transportable; a cooling system is required to control the reactor temperature because of the exothermic reaction of dimethylamine and carbon monoxide, and water, if present, can lead to catalyst decomposition and fouling of the reactor cooler through insoluble salt formation.
An alternative route to dimethylformamide, which has the added advantage of requiring no catalyst, is the batch or continuous reaction of dimethylamine with methyl formate.
The following patents illustrate various methods of preparing dimethylformamide; they are:
U.S. Pat. No. 2,866,822 (Siefen et al., 1958) discloses a process for producing formamides in a vertical reaction column. Carbon monoxide is passed through a dispersion plate at the bottom of the column and a methylamine, e.g., dimethylamine, and an alkali metal methylate catalyst in methanol solution are introduced at the top of the fractionation column. Off-gases pass from the reactor at the top of the reactor and products are withdrawn at the bottom.
U.S. Pat. No. 3,072,725 (Surman, 1963) discloses a continuous process for producing dimethylformamide wherein methyl formate and dimethylamine are reacted in a reaction zone. The reaction products are passed to a heated distillation zone (reactor-stripper) for vaporizing and effecting separation of the unreacted dimethylamine and methyl formate from the liquid products methanol and dimethylformamide. Unreacted methyl formate and dimethylamine are returned to the top of the reactor-stripper distillation column as reflux. Liquid products, e.g., all of the methanol and dimethylformamide, essentially free of methyl formate and dimethylamine, are removed from the bottom of the reactor-stripper distillation column and passed to a second distillation zone. The second distillation effects separation of water and methanol from the dimethylformamide.
U.S. Pat. No. 3,530,182 (Haynes et al., 1970) discloses a method for producing hydrocarbon formamides in an autoclave by the catalytic reaction of carbon dioxide, hydrogen, and primary or secondary amines in the presence of halogen-containing transition metal compounds. Dimethylformamide is prepared by reacting carbon dioxide, hydrogen and dimethylamine in the presence of dichloro-bis(triphenylphosphine)palladium.
U.S. Pat. No. 4,098,820 (Couteau et al., 1978) discloses a direct carbonylation process for producing dimethylformamide wherein carbon dioxide is reacted with a recycled liquid dimethylamine mixture and a methanolic solution of an alkali metal or earth metal methoxide catalyst. The recycled liquid mixture is used to draw in carbon dioxide gas and disperse the carbon dioxide gas within the reaction medium.
U.S. Pat. No. 4,853,485 (Bellis, 1989) discloses the formation of dimethylformamide at reasonable yield by the reaction of formamide and methanol in the presence of a quaternary ammonium catalyst.
RO 75982 (Serban et al., 1981) discloses a continuous process for the reaction of methanol and dimethylamine in the presence of an activated charcoal catalyst. The reaction mixture, which includes dimethylformamide, methyl formate, and methanol, are rectified by known means.
This invention relates to an improvement in a continuous process for the production of dimethylformamide. In the basic process, methyl formate and dimethylamine are reacted at a temperature and pressure sufficient to form dimethylformamide, generating by-product methanol. The improvement in the process resides in the steps:
(a) reacting methyl formate and dimethylamine in a reactive distillation column under conditions to form dimethylformamide and by-product methanol;
(b) vaporizing the by-product methanol and generating a liquid dimethylformamide while in said reactive distillation column;
(c) removing at least a major portion of the by-product methanol as an overhead from said reactive distillation column;
(d) removing a crude liquid dimethylformamide containing residual by-product methanol as a bottoms fraction from said reactive distillation column;
(e) introducing said bottoms fraction containing dimethylformamide and residual by-product methanol to a purification column wherein the by-product methanol is removed from said dimethylformamide as an overhead and purified dimethylformamide is removed as a bottoms fraction; and, optionally,
(f) recycling the by-product methanol removed as an overhead from the purification column to the reactive distillation column.
There are significant advantages that can be achieved by this process and these include:
an ability to integrate reaction of dimethylamine and methyl formate and the initial separation of the resulting product and by-product in a single piece of equipment;
an ability to integrate the heat generated in the reaction process within the reaction and distillation stages of a single column; and,
an ability to react methyl formate and dimethylamine in the distillation zone to produce dimethylformamide on a continuous basis without fouling of the distillation/separation stages.